What should the Roll of each bone be?
There aren't any rules for particular roll for particular bones.  For bones that rotate in only a single axis, fingers and such, I tend to make that the X axis simply because it makes it easy to set the roll from a cursor (select tail of bone and head of parent, snap cursor to selection, recalculate roll->cursor.)

A good way to build rigs to get easily mirrored poses is to build only one half of them and then use armature edit mode's "symmetrize" tool to create the other half.  This will provide appropriate rolls.

Another way to fix rolls is to turn on X-mirror in tool options and set the roll for one side's bones individually.  Blender recognizes bone symmetry on the basis of name, not roll, so x-mirror will apply roll changes appropriately even to bones that aren't fully mirrored (unmirrored roll.)